Diffuse interstitial lung disease in children can be classified in numerous ways. Classification by etiology and by age at presentation is particularly useful (Box 240.1). Age at presentation is an important factor when considering the diagnostic possibilities because a group of conditions exist that are seen only in newborns and infants but not in older children. Newborns and near-term infants who present with early respiratory failure and fail to respond to conventional therapies should be assessed for developmental and genetic disorders, including deficiency of surfactant proteins B and C and abnormality of ABCA3, a transporter protein thought to be involved in surfactant secretion, the structural abnormality known as alveolar capillary dysplasia with misalignment of pulmonary veins, congenital alveolar dysplasia, and other abnormalities of intrauterine lung growth. The disease processes that present exclusively in infants include persistent tachypnea of infancy (also known as neuroendocrine hyperplasia of infancy), infantile cellular interstitial pneumonitis (also known as pulmonary interstitial glycogenosis), and chronic pneumonitis of infancy (now widely thought to be the histologic appearance of some surfactant dysfunction mutations, more often surfactant C protein deficiency). Most cases of pulmonary alveolar proteinosis seen in children, as well as the histologic appearance of some of surfactant dysfunction mutations, also present in infants.

These diseases are all uncommon occurrences, and many are poorly understood. The usual clinical presentation is insidious
onset and minimal early findings, although acute fulminate presentation can occur. Tachypnea is the hallmark physical finding and may be the only early finding, although occasionally acute presentation with tachypnea, hypoxemia, and fever may occur. Older children may present with dyspnea, with or without wheezing, whereas infants and toddlers more frequently present with tachypnea without respiratory distress. Fine crackles often are present with progression of disease. Symptoms overlap with those of common respiratory infections, but the prolonged course, lack of response to initial therapies, and imaging findings will separate these processes from the more common acute pulmonary infections. The chest radiograph may be normal initially, but later it often shows a diffuse interstitial pattern of reticulonodular infiltrate. Chest computed tomography (CT) scan generally is a more sensitive indicator of disease activity and shows better correlation with symptoms than does chest radiograph. Separating the various disease processes that produce chronic diffuse interstitial lung disease may be difficult using clinical, laboratory, and radiologic findings, and frequently lung biopsy is necessary to define the pathologic process.